1. Technical Field
The present invention relates to an apparatus for removing trace amounts of toxic substances contained in exhaust gas resulting from the combustion of petroleum or coal, in particular, metallic mercury compounds, and a method of operating the same. Specifically, it relates to an apparatus for removing trace amounts of toxic substances from exhaust gas which is capable of removing metallic mercury compounds stably and efficiently after a prolonged operation thereof and a method of operating the same.
2. Background Art
Exhaust gas from combustion equipment such as a boiler using petroleum or coal etc., contains not only nitrogen oxides (NOx) and sulfur oxides (SOx) which are causative substances of photo-chemical smog and acid rain but also compounds of heavy metals such as metallic mercury as trace amounts of toxic substances. As an efficient method of removing NOx, a method of denitration of exhaust gas in which reducing agents such as ammonia (NH3) are used to effect selective catalystic reduction has been widely adopted mainly at thermal power plants. Mainly used catalysts are those in which vanadium (V), molybdenum (Mo) and tungsten (W) are used as active compositions and titanium oxide (TiO2) is used as a carrier. In particular, those in which vanadium is contained as one of active compositions have now been mainly used as denitration catalysts due to the fact that they are not only high in activity but also less likely to be deteriorated by impurities contained in exhaust gas and can be used at lower temperatures (Japanese Published Unexamined Patent Application No. S50-128681 and others). Further, the catalyst compositions are ordinarily formed and used in a honey-comb shape or a plate-shape structure, and methods for manufacturing them have been invented for various types.
On the other hand, regarding the removal of SOx contained in exhaust gas from the combustion equipment, a wet desulfurization unit in which limestone slurry is used to absorb and remove SOx in the exhaust gas is able to attain a highly efficient desulfurization. Therefore, this desulfurization unit has been mainly used for desulfurization. Independent of this unit, there has been proposed a semidry desulfurization unit in which lime or magnesium hydroxide (Mg (OH)2) is used as an absorbent. A desulfurization method using the semidry desulfurization unit is a method in which an absorbent such as limestone is directly sprayed into exhaust gas inside a flow channel of exhaust gas on the upstream side of a dust removal unit and retained inside the flow channel of exhaust gas or the dust removal equipment for a predetermined period of time after the spraying, thereby removing SOx from the exhaust gas. This method is advantageous in that it is economical because of the lower cost of the equipment, although not suitable for attaining a highly efficient desulfurization.
Meanwhile, over the last few years, studies have been actively conducted for reducing the exhaust of metallic mercury compounds in exhaust gas resulting from the combustion of petroleum or coal. The metallic mercury compounds are to seriously affect humans through the food chain once they are emitted into the atmosphere. Regarding trace amounts of toxic substances such as metallic mercury resulting from the combustion of petroleum or coal, compositions evaporated by the combustion are moved into exhaust gas. In most cases, metallic mercury is said to be exhausted as gaseous metallic mercury at a combustion zone in the vicinity of 1,500° C. It has been confirmed that metallic mercury is partially oxidized by concurrent hydrogen chloride (HCl) to give mercury chloride (HgCl2) in a relatively low temperature region (from 300 to 450° C.) of a flow channel of exhaust gas as shown by the following formula (1). It is also known that the reaction is facilitated on a denitration catalyst set in a temperature range of approximately 300° C. to 450° C. Further, it is known that the reaction proceeds substantially completely in the rightward direction at a temperature lower than 300° C. and metallic mercury is oxidized to mercury chloride.Hg+HCl+1/2O2=HgCl2+H2O  (1)
Mercury chloride (HgCl2) generated in the above reaction formula (1) is lower in steam pressure than metallic mercury, absorbed on dust and removed by a dust removal unit disposed on the downstream side of the flow channel of exhaust gas. Further, it is known that since mercury chloride (HgCl2) is easily absorbed by water, it is absorbed by an absorption liquid such as limestone slurry in a wet desulfurization unit or absorbed and removed by a spray absorbent in a semidry desulfurization unit.
However, there are fears that metallic mercury kept unoxidized may be exhausted mostly from smoke stacks in a gaseous state (metallic mercury vapor) as it is.
Thus, as technologies for decreasing an exhausted amount of metallic mercury, there have been proposed several methods: that is, a method (conventional technology 1) in which activated carbon is sprayed into a flow channel of exhaust gas on the upstream side of dust removal equipment disposed at a low temperature region to remove metallic mercury effectively by the absorption effect and catalyst effect of the activated carbon; a method (conventional technology 2) in which, as shown in the invention described in Japanese Published Unexamined Patent Application No. 2003-53142, a solid oxidation catalyst layer is disposed in a low temperature region (300° C. or lower) on the downstream side of a heat exchanger in a flow channel of exhaust gas where a denitration catalyst, an air preheater, a dust removal unit and the heat exchanger are disposed in sequence from the upstream side, thereby oxidizing metallic mercury and then absorbing and removing the metallic mercury by an absorbing liquid in a wet desulfurization unit; and a method (Japanese Published Unexamined Patent Application No. H10-66814, conventional technology 3) using an exhaust gas-treating bag filter in which a metallic mercury-absorbing agent is retained on a filter cloth for removing toxic substances contained in exhaust gas.    Patent Document 1: Japanese Published Unexamined Patent Application No. 2003-53142    Patent Document 2: Japanese Published Unexamined Patent Application No. H10-66814